1. Field of the Invention
The present invention relates to an information recording medium on and from which information can be recorded and reproduced by means of a light beam, and particularly to a recording medium which is capable of achieving a high track density and a recording/reproducing apparatus therefor.
2. Description of the Related Art
Optical disk, optical card and optical tape have been known as the media capable of recording or reproducing information by means of optical processes. On most of these media, information is recorded by applying a laser beam focused to a tiny spot by means of a lens onto a thin recording film of the media. An optical disk, among them, has a disk substrate having guide tracks comprising bumps and recesses being formed in a spiral or concentric configuration and a thin recording film formed thereon. In such a disk, information is recorded or reproduced by irradiating the laser beam along the track.
A conventional optical disk will be described below with reference to FIGS. 17(a)-(c) and 18(a)-(c). Recording signals on a recording medium or reproducing signals therefrom require control information to search for a recordable area on the recording medium or to locate the position where desired information is recorded. For this purpose, an optical disk has an address area to control the information recording area for a specified period of the track on the disk. The address area may be either of two types; recording the information signals on bumps of the guide track, or recording them in recesses. Faces projecting toward the incident light beam are called grooves, and recording on the projecting faces is called groove recording. Faces which are recessed are called lands, and recording on the recessed faces is called land recording. FIGS. 17(a)-(c) and 18(a)-(c) illustrate typical constitutions of the respective types.
FIG. 17(a) shows an enlarged example of a vicinity of an address area of a recording medium used in groove recording, while FIG. 17(b) showing a cross sectional view of the medium taken along a line 17(b)--17(b) of FIG. 17(a), and FIG. 17(c) showing a cross sectional view of the center of a recording track taken along a line 17(c)--17(c) of FIG. 17(a). As shown in FIG. 17(a), the recording medium includes an address area 2 and an information recording area (hereinafter, called information area) 3. The address area 2 is formed on information guide tracks separated from the information area 3. As shown in FIG. 17(b) and 17(c), the address area 2 and the information area 3 consist of grooves 141 and lands 142. A recording mark 7 corresponding to the information signal to be recorded is formed on the groove 141. Address pits 143 representing the address signal are provided in such a manner as to interrupt the grooves 141.
FIG. 18(a) shows a pattern of address pits in the case of land recording, while FIG. 18(b) showing a cross sectional view of the medium taken along a line 18(b)--18(b) of FIG. 18(a), and FIG. 18(c) showing a cross sectional view of the center of a recording track taken along a line 18(c)--18(c) of FIG. 18(a). This recording medium also includes an address area 2 and an information recording area 3 separated therefrom. The address area 2 and the information area 3 consist of grooves 151 and lands 152. In this case, recording marks 153 are formed on the recessed land 152 with respect to the laser beam 1. In the address area 2, address pits 154 are formed in the form of bumps and recesses in the same track as the recording marks 153.
As shown in FIGS. 17(a)-(c) and 18(a)-(c), address pits are formed on the center line of the track wherein information signals are recorded, in either of land recording and groove recording. When demodulating the information signals, changes in the intensity of the light reflecting on the address pits are demodulated as the address information to locate the recording position of the information signal on the optical disk, thereby enabling it to record and reproduce the information at a particular position.
A method of manufacturing a substrate provided with address pits will be described below with reference to FIGS. 19(a)-(c). FIG. 19(a) shows a flow chart of the method and FIG. 19(b) shows a schematic view illustrating the processes of groove recording. FIG. 19(c) shows a schematic view illustrating the processes of land recording, which is different from those of groove recording. The manufacturing process includes a mastering process to make a master disk having an inverted impression of the required substrate surface configuration and a replication process to form substrates from the master disk.
The mastering process will be described below. First, a flat glass plate 161 is coated with a photoresist 162 and is rotated while a spiral pattern is printed thereon by exposing the plate 161 to an Ar laser beam. For the groove recording shown in FIG. 17(a), for example, a single Ar laser beam 163 of a constant output power is applied to form the guide tracks of the information recording section, as shown in FIG. 19(b). In the address area, the laser power is modulated in a specified pattern to expose the area corresponding to the address to the laser beam 163. In land recording, as shown in FIG. 19(c), two laser beams 164 and 165 spaced by 1/2 of the track pitch in the tracking direction are used. The first laser beam 164 is used for forming the tracks while the second laser beam 165 is modulated in the specified pattern to record the address signal. Then, the photoresist 162 is removed from the exposed portions 166, 167 and 168 in a development process. A nickel layer 169 is formed on the surface in a plating process, and finally the nickel layer 169 is peeled off the glass plate 161, thereby obtaining a master disk 170 which has bumps and recesses on the surface.
The replication process will be described below. Various methods are used according to the substrate material. To manufacture optical disks, an injection molding method is predominantly employed because it is suited to mass production. In the injection molding method, the master disk 170 obtained in the mastering process is set in a mold 171 installed in an injection molding machine and a resin 172 is injected therein, thereby obtaining a resin substrate 173 having the specified tracks of bumps and recesses. A thin recording film is formed on the resin substrate to obtain an optical disk as a recording medium capable of recording information.
As described above, address areas can be formed for the control information of the recording medium capable of optically recording and reproducing information. However, using such optical disks in wide applications such as recording large amount of data or image information requires a higher recording density. Recording density may be increased by employing a laser of a shorter wavelength or a light converging lens having a larger numerical aperture. Because this method enables making the light spot smaller, it is capable of not only increasing the recording density in the direction of the track but also decreasing the track pitch. The optical disk currently in use has a track pitch of Tp=1.6 .mu.m and a guide track width about a half thereof with a depth of about 50 nm. However, when the light spot is made smaller and the track pitch is accordingly decreased to 1.0 .mu.m or less, for example, for higher recording density, such problems as described below arise.
A master disk having a track pitch of 1.0 .mu.m or less can be made in the mastering process described above. It is also possible to increase the density further by employing a laser of a shorter wavelength to which the photoresist is exposed. The requirement for the configuration of the substrate in view of the recording characteristics in the case of a small track pitch of the guide track is to make the track width in a portion, where the recording marks are formed, as large as possible to maintain the signal amplitude. Therefore, groove width is made larger on a substrate for groove recording, and the land width is made larger on a substrate for land recording. However, transfer performance of the injection molding process becomes poorer as the track pitch becomes smaller. The transfer performance refers to the degree of accuracy in reproducing the surface configuration of the master disk onto the resin substrate obtained by the injection molding. In the injection molding process, molten resin is poured into a mold and the configuration of the master disk is transferred onto the resin substrate by means of the injection pressure. Thus, when the width of the recess of the master disk, namely the land, wherein the resin is injected is decreased, the transfer performance becomes poorer.
Problems which arise when the track pitch is decreased will be described below with reference to FIG. 20 which illustrates a cross sectional view of an injection molding machine. A substrate for groove recording will be taken as an example. When the track pitch Tp is decreased while maintaining the groove width Gw, narrower land between bumps, namely the track 174 on the master disk, must be filled with the resin as shown in FIG. 20. This requires large equipment having a very high injection pressure.
Similarly on a substrate for land recording, smaller track pitch causes the width of the land area 155 on both sides of the address pit shown in FIG. 18 to become excessively small, making the injection molding difficult.